Glossary

4.1 Evidence for dark matter

7 min read•august 20, 2024

Dark matter, a mysterious form of matter that doesn't interact with light, plays a crucial role in shaping our universe. Scientists have gathered compelling evidence for its existence through observations of , , and effects.

This invisible matter forms halos around galaxies and clusters, influencing their formation and evolution. Its presence explains the observed structure of the universe and has profound implications for our understanding of cosmic history and future.

Observational evidence for dark matter

  • Dark matter's existence inferred through its gravitational effects on visible matter, radiation, and the structure of the universe
  • Multiple independent observations across a wide range of scales provide compelling evidence for the presence of dark matter
  • Observational evidence includes galactic rotation curves, velocity dispersions in galaxies, and gravitational lensing effects

Galactic rotation curves

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  • Rotation curves of spiral galaxies show flat velocity profiles at large radii, contrary to expectations based on visible matter alone
  • Observed velocities of stars and gas in the outer regions of galaxies are much higher than predicted by Newtonian dynamics
  • Presence of a surrounding galaxies explains the flat rotation curves (Milky Way, Andromeda)
  • increases with distance from the galactic center, indicating the presence of non-luminous matter

Velocity dispersions in galaxies

  • Velocity dispersion measures the spread of velocities about the mean velocity for a group of astronomical objects (stars in galaxies, galaxies in clusters)
  • Observed velocity dispersions in elliptical galaxies and galaxy clusters are higher than expected based on the visible matter
  • Presence of dark matter provides additional gravitational potential to explain the high velocity dispersions (Coma Cluster, Virgo Cluster)
  • Virial theorem relates the kinetic energy of a system to its gravitational potential energy, allowing the estimation of total mass

Gravitational lensing effects

  • Massive objects distort spacetime, causing light rays to bend and leading to gravitational lensing
  • Strong lensing creates multiple images, arcs, or Einstein rings around massive galaxy clusters (Abell 1689, Abell 2218)
  • Weak lensing causes small distortions in the shapes of background galaxies, allowing the mapping of dark matter distribution
  • Gravitational lensing provides a direct probe of the total mass distribution, independent of the nature of matter (visible or dark)

Theoretical basis for dark matter

  • Dark matter is a hypothetical form of matter that interacts gravitationally but does not emit, absorb, or scatter electromagnetic radiation
  • Theoretical models and simulations of structure formation require the presence of dark matter to explain observations
  • Particle physics provides several candidates for dark matter particles, although their exact nature remains unknown

Cosmological models and dark matter

  • The Lambda Cold Dark Matter (ΛCDM) model is the standard cosmological model, incorporating dark matter and dark energy
  • Cold dark matter consists of slow-moving, non-relativistic particles that interact weakly with ordinary matter
  • Dark matter plays a crucial role in the formation and evolution of structure in the universe, from galaxies to galaxy clusters and
  • Cosmological simulations (Millennium Simulation, Bolshoi Simulation) demonstrate the importance of dark matter in reproducing the observed

Particle physics candidates

  • Weakly Interacting Massive Particles () are a leading candidate for dark matter, with masses ranging from a few GeV to several TeV
  • WIMPs are thought to be thermal relics from the early universe, with an abundance that matches the observed dark matter density (WIMP miracle)
  • (neutralinos, gravitinos) and extra-dimensional Kaluza-Klein particles are some of the proposed WIMP candidates
  • , ultra-light scalar particles, are another promising dark matter candidate, with masses much lower than WIMPs (10−610^{-6} to 10−310^{-3} eV)

Dark matter distribution

  • Dark matter is believed to form halos surrounding galaxies and to be present in galaxy clusters and on larger cosmological scales
  • The distribution of dark matter influences the formation and evolution of structures in the universe
  • Numerical simulations and observations help constrain the dark matter distribution across various scales

In galaxies and galaxy clusters

  • Dark matter halos extend well beyond the visible components of galaxies, with a density profile described by the Navarro-Frenk-White (NFW) or Einasto models
  • The dark matter halo of the Milky Way is estimated to have a mass of ∼1012\sim10^{12} solar masses and a radius of ∼200\sim200 kpc
  • Galaxy clusters contain a significant amount of dark matter, with a mass fraction of ∼85%\sim85\% (, Abell 520)
  • The Bullet Cluster provides direct evidence for dark matter through the separation of the gravitational lensing signal from the during a cluster collision

On cosmological scales

  • Dark matter forms a cosmic web, with galaxies and clusters residing in filaments and sheets surrounding large voids
  • The power spectrum of matter fluctuations, derived from cosmic microwave background (CMB) and large-scale structure surveys, is consistent with the presence of cold dark matter
  • (BAO) in the matter power spectrum provide a standard ruler for measuring the expansion history of the universe and constraining dark matter properties
  • The , absorption features in the spectra of distant quasars, probes the distribution of matter on scales smaller than galaxies

Alternatives to dark matter

  • Some alternative theories attempt to explain the observational evidence without invoking dark matter
  • These theories modify the laws of gravity on galactic and cosmological scales, aiming to reproduce the observed phenomena
  • The most prominent alternatives are (MOND) and (TeVeS)

Modified Newtonian dynamics (MOND)

  • MOND proposes a modification to Newton's second law of motion at low accelerations (a<a0≈10−10a < a_0 \approx 10^{-10} m/s2^2)
  • In the MOND regime, the gravitational acceleration is given by a=aNa0a = \sqrt{a_N a_0}, where aNa_N is the Newtonian acceleration
  • MOND successfully explains the flat rotation curves of galaxies without the need for dark matter
  • However, MOND faces challenges in explaining the observations on larger scales, such as galaxy clusters and the CMB power spectrum

Tensor-vector-scalar gravity (TeVeS)

  • TeVeS is a relativistic generalization of MOND, introducing additional fields (tensor, vector, and scalar) to the gravitational action
  • The theory aims to reproduce the successes of MOND on galactic scales while remaining consistent with general relativity on larger scales
  • TeVeS can explain gravitational lensing observations without dark matter, as the additional fields can mimic the effect of dark matter
  • However, TeVeS still struggles to fully account for observations on cluster and cosmological scales, and it introduces additional complexity compared to the standard dark matter model

Experimental searches for dark matter

  • Experimental efforts aim to detect dark matter particles through their interactions with ordinary matter or their annihilation or decay products
  • Searches for dark matter are conducted using both direct and indirect detection methods
  • The detection of dark matter particles would provide crucial insights into the nature of dark matter and its role in the universe

Direct detection experiments

  • Direct detection experiments aim to observe the elastic scattering of dark matter particles off atomic nuclei in sensitive detectors
  • These experiments typically use large, ultra-pure crystals (germanium, silicon) or noble liquids (xenon, argon) as target materials
  • Examples of direct detection experiments include XENON1T, LUX, PandaX, SuperCDMS, and CRESST
  • The experiments are located deep underground to minimize background noise from cosmic rays and require careful shielding and background reduction techniques

Indirect detection methods

  • Indirect detection methods search for the products of dark matter annihilation or decay, such as gamma rays, neutrinos, and cosmic rays
  • The Fermi Gamma-ray Space Telescope searches for gamma-ray signals from dark matter annihilation in dwarf galaxies and the Galactic Center
  • The IceCube Neutrino Observatory looks for high-energy neutrinos from dark matter annihilation in the Sun or the Galactic Center
  • The Alpha Magnetic Spectrometer (AMS-02) on the International Space Station searches for antimatter and other cosmic rays that could be produced by dark matter annihilation
  • Indirect detection methods rely on understanding the astrophysical backgrounds and the dark matter distribution in the target regions

Implications of dark matter

  • The existence of dark matter has profound implications for our understanding of and evolution, as well as the large-scale structure of the universe
  • Dark matter plays a crucial role in shaping the cosmic web and the properties of galaxies and galaxy clusters
  • The nature of dark matter also has important consequences for the evolution of the universe and its ultimate fate

For galaxy formation and evolution

  • Dark matter halos provide the gravitational potential wells in which galaxies form and evolve
  • The properties of dark matter halos (mass, size, concentration) influence the properties of the galaxies that form within them (Tully-Fisher relation, Faber-Jackson relation)
  • Dark matter halos undergo hierarchical growth, with smaller halos merging to form larger ones, driving the evolution of galaxies through mergers and accretion
  • The feedback processes between baryonic matter and dark matter (adiabatic contraction, feedback from star formation and active galactic nuclei) shape the final properties of galaxies

For the large-scale structure of the universe

  • Dark matter is the dominant component of matter in the universe, making up ∼85%\sim85\% of the total matter content
  • The large-scale structure of the universe, as observed through galaxy surveys (Sloan Digital Sky Survey, Dark Energy Survey), is determined by the distribution of dark matter
  • Dark matter halos are the building blocks of the cosmic web, with galaxies and clusters residing in the nodes and filaments of the web
  • The growth of structure in the universe, as described by the matter power spectrum and the halo mass function, depends on the properties of dark matter (cold vs. warm, interaction cross-section)
  • The nature of dark matter has implications for the future evolution of the universe, influencing the growth of structure and the eventual fate of the cosmos (Big Freeze, Big Rip, Big Crunch)

Key Terms to Review (25)

Axions: Axions are hypothetical elementary particles that are proposed as a solution to the strong CP problem in quantum chromodynamics and are also considered a candidate for dark matter. These elusive particles would have very low mass and interact very weakly with ordinary matter, making them difficult to detect. Their existence could help explain some of the phenomena observed in galaxies and the universe that suggest the presence of dark matter.
Baryon Acoustic Oscillations: Baryon acoustic oscillations refer to the periodic fluctuations in the density of visible baryonic matter (normal matter) in the universe, which were produced by sound waves in the early universe. These oscillations are critical as they provide evidence of the distribution of matter and energy in the cosmos, influencing structures like galaxy clusters, superclusters, and voids.
Baryonic Matter: Baryonic matter refers to the type of matter composed of baryons, which are particles made up of three quarks, including protons and neutrons. This form of matter makes up all the visible structures in the universe, such as stars, planets, and galaxies, and plays a crucial role in understanding the overall composition of the universe, especially when discussing dark matter and its effects on galaxy formation and dynamics.
Bullet Cluster: The Bullet Cluster is a pair of colliding galaxy clusters that provides compelling evidence for the existence of dark matter. Observations of this cosmic collision reveal that most of the mass is not visible in the form of stars or gas but is instead attributed to an unseen substance, reinforcing the theory that dark matter makes up a significant portion of the universe's mass.
Computer simulations: Computer simulations are computational models that use algorithms to replicate the behavior of complex systems over time, allowing researchers to visualize and analyze phenomena that may be difficult or impossible to observe directly. They are essential tools in fields like astrophysics, where they help in understanding the formation and evolution of structures in the universe, including dark matter and the concept of a multiverse.
Cosmic Microwave Background Radiation: Cosmic microwave background radiation (CMB) is the faint glow of microwave radiation that fills the universe, a relic from the early stages of the universe shortly after the Big Bang. This radiation provides critical evidence for various cosmological theories, serving as a key element in understanding dark matter, cosmic inflation, primordial nucleosynthesis, and the expansion of the universe.
Cosmic Web: The cosmic web is the large-scale structure of the universe, characterized by a vast network of galaxies, clusters, and superclusters interconnected by filaments of dark matter and gas, along with vast voids in between. This intricate structure highlights the distribution of matter and energy in the universe and plays a crucial role in understanding its formation and evolution.
Dark matter halo: A dark matter halo is a theoretical region surrounding galaxies, composed of dark matter that does not emit light or energy, making it invisible to direct observation. This halo is believed to contain most of the mass in a galaxy and plays a crucial role in its gravitational dynamics and structure. The presence of dark matter halos helps to explain various phenomena in the universe, such as galaxy formation, the behavior of galaxies within clusters, and the movement of stars within galaxies.
Einasto Model: The Einasto model is a mathematical representation used to describe the density profile of dark matter halos in galaxies. It is characterized by a smooth and continuous decline in density with increasing distance from the center of the halo, reflecting the influence of dark matter on the structure and dynamics of galaxies. This model has gained attention for providing a better fit to observational data compared to other models, such as the Navarro-Frenk-White (NFW) profile.
Fritz Zwicky: Fritz Zwicky was a Swiss astrophysicist known for his groundbreaking contributions to the understanding of dark matter and galaxy dynamics. He proposed the existence of dark matter in the 1930s based on his observations of galaxy clusters, most notably the Coma Cluster. His work laid the foundation for later research that would reveal the critical role dark matter plays in the structure and behavior of galaxies.
Galactic rotation curves: Galactic rotation curves are graphs that show the relationship between the orbital speed of stars and gas in a galaxy and their distance from the galaxy's center. These curves reveal that stars farther from the center move at similar speeds to those closer in, challenging classical mechanics and suggesting the presence of unseen mass, which is primarily attributed to dark matter. The unexpected shape of these curves is key to understanding the dynamics of galaxies and their overall mass distribution.
Galaxy formation: Galaxy formation is the process through which galaxies evolve from small density fluctuations in the early universe into the vast structures we observe today. This complex process involves gravitational interactions, dark matter, star formation, and cosmic evolution, contributing to the diverse types of galaxies seen in the universe. Key factors in galaxy formation include the role of dark matter in shaping galaxy structures, the creation and dynamics of HII regions where new stars are born, the application of the virial theorem to understand galaxy stability, and how galaxy mergers influence their growth and morphology.
Gravitational Lensing: Gravitational lensing is a phenomenon that occurs when a massive object, such as a galaxy or a cluster of galaxies, bends the light from a more distant object due to its gravitational field. This effect not only magnifies and distorts the image of the background object but can also provide crucial information about the mass and distribution of dark matter in the lensing object, connecting it to various cosmic structures and dynamics.
Lambda cold dark matter model: The lambda cold dark matter model, often abbreviated as \( \Lambda CDM \), is the prevailing cosmological model that describes the large-scale structure and evolution of the universe. It combines the effects of a cosmological constant, denoted by \( \Lambda \), which represents dark energy, with cold dark matter, a non-baryonic form of matter that interacts only through gravity, playing a crucial role in shaping cosmic structures like galaxies and clusters.
Large-scale structure: Large-scale structure refers to the organization and distribution of matter in the universe on scales larger than individual galaxies, encompassing clusters, superclusters, and the cosmic web. This framework helps us understand how galaxies and other cosmic structures form and evolve under the influence of gravitational forces and dark matter.
Lyman-alpha forest: The Lyman-alpha forest refers to a series of absorption lines in the spectra of distant quasars and galaxies caused by neutral hydrogen clouds along the line of sight. These absorption features occur at the Lyman-alpha wavelength of 121.6 nm and indicate the presence of intervening hydrogen gas in the universe, providing insight into the distribution and properties of baryonic matter as well as the large-scale structure of the cosmos.
Mass-to-light ratio: The mass-to-light ratio is a key astronomical measurement that compares the total mass of an astronomical object, like a galaxy, to its luminosity, or the amount of light it emits. This ratio helps in understanding the distribution of matter, including both visible and dark matter, within galaxies and clusters, highlighting discrepancies between mass estimates derived from gravitational effects and those inferred from light output.
Modified Newtonian Dynamics: Modified Newtonian Dynamics (MOND) is a theoretical framework that alters Newton's laws of motion and gravitation to account for the observed behavior of galaxies without invoking dark matter. MOND proposes that at very low accelerations, such as those found in the outskirts of galaxies, the gravitational force behaves differently than predicted by traditional Newtonian physics, which has implications for how we understand galaxy rotation curves and the distribution of mass in galaxies.
Navarro-Frenk-White Model: The Navarro-Frenk-White (NFW) model describes the density profile of dark matter halos surrounding galaxies. This model provides a mathematical framework for understanding how dark matter is distributed, suggesting that the density of dark matter increases as one approaches the center of the halo and decreases with distance from it. The NFW model is crucial for explaining the gravitational effects observed in galaxies, supporting the existence of dark matter.
Observational Astronomy: Observational astronomy is the branch of astronomy that involves the collection and analysis of data from astronomical objects through the use of telescopes and other instruments. This practice allows scientists to study celestial phenomena and gather evidence to understand the universe's structure, dynamics, and composition. Observational astronomy is vital for testing theories in cosmology, including the existence of dark matter and the implications of multiverse theories.
Supersymmetric Particles: Supersymmetric particles are theoretical counterparts to the known particles in the Standard Model of particle physics, predicted by supersymmetry, which posits a symmetry between bosons and fermions. These particles have not yet been observed but are considered crucial for explaining phenomena such as dark matter, as they could provide candidates for the elusive particles that make up this mysterious component of the universe.
Tensor-Vector-Scalar Gravity: Tensor-Vector-Scalar (TeVeS) gravity is a modified theory of gravity that extends general relativity by incorporating additional fields: a vector field and a scalar field alongside the traditional tensor field. This framework aims to address phenomena that cannot be explained by general relativity alone, particularly the behaviors associated with dark matter and dark energy. The inclusion of these extra fields allows for a richer understanding of gravitational interactions and provides alternative explanations for observations such as galaxy rotation curves.
Velocity Dispersions: Velocity dispersions refer to the range of velocities of stars or other celestial objects within a specific system, indicating how much their speeds vary from the average. This variation can provide critical insights into the mass distribution of galaxies and clusters, helping to infer the presence of dark matter and trace the history of star formation in a galaxy.
Vera Rubin: Vera Rubin was an influential American astronomer known for her pioneering work on the rotation curves of galaxies, which provided critical evidence for the existence of dark matter. Her groundbreaking observations helped reshape our understanding of galaxy dynamics and the structure of the universe, connecting closely to various concepts including galaxy formation and the cosmic web.
WIMPs: WIMPs, or Weakly Interacting Massive Particles, are hypothetical particles that are proposed as a primary candidate for dark matter. These particles would interact through the weak nuclear force and gravity but would not interact with electromagnetic forces, making them difficult to detect. WIMPs play a significant role in explaining the evidence for dark matter, as their existence could help account for the gravitational effects observed in galaxies and clusters.
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